In a mobile communication system, an OFDM scheme using frequency hopping is under study. An OFDM system using frequency hopping uses different hopping patterns among a plurality of cells so as to carry out communications by averaging interference among cells.
That is, when two neighboring cells A, B as shown in FIG. 1 are considered, a base station BSA in the cell A and a base station BSB in the cell B transmit OFDM signals with different hopping patterns. Since these hopping patterns are normally determined at random in the cell A and cell B, there is a possibility that collision may occur by accident on a certain subcarrier at a certain time point.
This will be explained using FIG. 2. FIG. 2 shows a frequency hopping OFDM signal transmitted from the base station BSA in the cell A and a frequency hopping OFDM signal transmitted from the base station BSB in the cell B. One unit on the vertical axis shows a subcarrier and one unit on the horizontal axis shows a 1-burst period. That is, suppose one OFDM symbol is arranged in one square in the figure.
As is also clear from FIG. 2, an OFDM signal in the cell A collides with an OFDM signal in the cell B by accident on a certain subcarrier at a certain time point. As shown in FIG. 3, reception quality of a data symbol placed on the subcarrier at the time of collision deteriorates compared to other data symbols.
Thus, in an OFDM system using frequency hopping, the quality of symbols affected by interference from other cells deteriorates, and therefore it is necessary to carry out error correcting processing at the time of decoding to correct the data of the symbols whose quality is deteriorated to correct decoded data.
Here, a normal error correcting code is designed to carry out error correction on assumption that the communication path is affected by white Gaussian noise. However, in a system like OFDM using frequency hopping, its communication path does not show white Gaussian noise but it shows a state in which impulse-like noise is added. For this reason, there is a problem that its error correcting performance deteriorates.
In order to correctly decode the signal affected by this impulse-like noise, there is also a code such as a Reed-Solomon code which is used to carry out error correcting processing by regarding symbols of poor reception quality as having been lost. However, even when a Reed-Solomon code is used, it is necessary to correctly notify the error correcting decoding section of symbols of poor reception quality.
In order to obtain decoded data having a good error rate characteristic from such a frequency hopping OFDM signal, it is necessary to correctly detect symbols involved in a collision. As one of such methods for detecting symbols, use of the method disclosed in the Unexamined Japanese Patent Publication No. HEI 11-252040 can be considered.
The technology described in the above Publication detects a subcarrier affected by interference by monitoring the state of a pilot signal placed on a specific subcarrier of an OFDM signal. Then, by carrying out weighting processing such as loss correction during error correcting processing according to the detection result, the above technology obtains decoded data with a good error rate characteristic even when interference occurs.
However, even if an attempt is made to detect the collision position on a frequency-hopped OFDM signal using the conventional interference detection apparatus described in the above Publication, it is not possible to detect the position of a data symbol on which the collision has occurred. This is because while a pilot signal is a signal placed on a predetermined subcarrier at a predetermined timing, a collision of a data symbol occurs on a subcarrier and at a timing which cannot be predicted from the pilot signal.
Therefore, it is possible to consider a method for detecting a symbol of poor quality as a symbol on which a collision has occurred by directly measuring the reception quality of a data symbol. However, since the data symbol is not a known signal, it is not possible to measure the reception quality (e.g., SIR (Signal to Interference Ratio)) of the data symbol.
Thus, the conventional OFDM system using frequency hopping cannot correctly detect a data symbol on which a collision has occurred. As a result, there is a problem that the error rate characteristic of decoded data deteriorates.
Furthermore, this problem is not limited to the OFDM system using frequency hopping, but can also occur, for example, in an OFDM system having a frequency scheduler. That is, this type of OFDM system is designed to measure the channel quality of each subcarrier in each cell, place a data symbol on a subcarrier of good channel quality and transmit it. But in such a case, there is also a possibility that a collision may occur on a certain subcarrier between neighboring cells and cause deterioration of the error rate characteristic of decoded data.